Nitride semiconductor device and fabrication method thereof

ABSTRACT

In a wafer having an LD structure  251  formed on a GaN-based substrate  250 , cleavage guide grooves  252  are formed in its surface by scribing from above the LD structure  251  with a diamond needle. The cleavage guide grooves  252  are formed one along each of stripe-shaped waveguides  253  formed parallel to the &lt; 1 - 100 &gt;direction of the wafer, and are formed in the shape of broken lines in the &lt; 11 - 20 &gt;direction of the wafer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride semiconductor device for usein a laser diode or light-emitting diode that emits blue light and to amethod of fabricating such a nitride semiconductor device. Moreparticularly, the present invention relates to a nitride semiconductordevice having a nitride semiconductor substrate and to a method offabricating such a nitride semiconductor device.

2. Description of the Prior Art

Expectations have been running high for and various applications havebeen attempted with Ill-V nitride semiconductors composed of a group IIIelement such as Al, Ga, or In and a group V element N (hereinafter sucha semiconductor will be referred to as a “GaN-based semiconductor”) aslight-emitting devices and power devices for their desirable bandstructure and chemical stability. For example, many attempts have beenmade to lay a layer of a GaN-based semiconductor on a sapphire substrateto produce a nitride semiconductor device that emits blue laser. Ingeneral, in AlGaInAs- or AlGaInP-based nitride semiconductor devices,cavities, which are essential for laser oscillation, are produced by theuse of cleavage planes.

However, in a case where a GaN-based semiconductor layer is laid on asapphire substrate, since sapphire does not cleave easily, the endsurfaces of the produced chip have surface irregularities as large as 4to 10 nm on average, making it difficult to obtain a satisfactorycavity. Furthermore, in a case where a nitride semiconductor device isformed by laying a GaN-based semiconductor layer on a sapphiresubstrate, since, in general, the angle at which sapphire cleaves is 30°apart from the angle at which the GaN-based semiconductor layer laid onthe substrate cleaves, it is difficult to reduce the surfaceirregularities on the end surfaces irrespective of along which of thesubstrate's and the upper layer's cleavage planes the chip is dicedapart.

For these reasons, much attention has been paid to using, as a substrateon which to lay a GaN-based semiconductor layer, a GaN-based substratethat cleaves easily and that cleaves in the same direction as theGaN-based semiconductor layer laid on its surface and producing the endsurfaces by cleavage. Here, a GaN-based substrate denotes a substrateformed out of a GaN-based semiconductor. When a GaN-based substrate isused, the GaN-based semiconductor layer and the GaN-based substratecleave in the same direction, and therefore the end surfaces areexpected to be flat. Moreover, when a GaN-based substrate is used, goodlattice matching is achieved between the GaN-based substrate and theGaN-based semiconductor layer laid on it, and no difference existsbetween their thermal expansion coefficients. This helps reduce thestrain on and hence defects of the nitride semiconductor device, and isthus expected to extend the useful life of the nitride semiconductordevice.

An example of a nitride semiconductor device in which, as describedabove, a GaN-based semiconductor layer is laid on a GaN-based substrateand then the end surfaces of the cavity are produced by cleavage isdisclosed, for example, in Japanese Patent Application Laid-Open No.H11-4048.

However, concerning the abovementioned example of a nitridesemiconductor device using a GaN-based substrate disclosed in JapanesePatent Application Laid-Open No. H11-4048, there is given no detaileddescription about how the end surfaces of the cavity are produced or howthe chip is diced apart. This has led the inventors of the presentinvention to try in various ways how a wafer using a GaN-based substratecleaves, only to find that, in practice, it is difficult to dice such awafer with a constant, uniform cavity length and at a satisfactory yieldrate.

For example, in the dicing process of a wafer 130, as shown in FIG. 13,having stripe-shaped optical waveguides 131 and having cleavage guidegrooves 132 formed at an edge so as to run in the direction of cleavage,ideally, cavities are produced as a result of the wafer 130 beingcleaved along cleaving lines, like the one designated as 133, running inthe same direction as the cleavage guide grooves 132. Cleaving alongsuch cleaving lines 133 permits the stripe-shaped optical waveguides 131to be split with flat surfaces. This makes it possible to producenitride semiconductor devices at a high yield rate. In reality, however,many dicing lines meander, like the one designated as 134, or run at60°, like the one designated as 135, relative to the desired dicingdirection.

One cause of the formation of such unintended dicing lines as thosedesignated as 134 and 135 is that, even when cleavage guide grooves 132are formed in the <11-21> direction (of which a description will begiven later) in which cleavage occurs, in a GaN-based substrate, whichhas a hexagonal crystal structure, directions that are at 60° relativeto that direction are equally valid cleavage directions, and thereforecleavage occurs as easily also along lines that are at 60° relative tothe desired dicing direction. If such unintended cleavage occurs onlyonce, a dicing line like the one designated as 135 in FIG. 13 results;if such unintended cleavage occurs continuously, a dicing line like theone designated as 134 results. Another cause is that, compared with asapphire substrate, a GaN-based substrate is so brittle as to makecleavage in inclined directions as described above more likely, causing,in the worst case, the devices to be broken to pieces.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a nitride semiconductordevice that is diced apart with flat surfaces at the ends of the cavity.Another object of the present invention is to provide a method offabricating a nitride semiconductor device which permits dicing to beachieved in a fixed direction all the time.

To achieve the above objects, according to one aspect of the presentinvention, a nitride semiconductor device is provided with:

a substrate that exhibits cleavage;

a nitride semiconductor layer including a cleavage plane equal to acleavage plane of the substrate and formed out of a compound containinga group III element and nitrogen;

a stripe-shaped optical waveguide formed in the nitride semiconductorlayer;

a cavity formed by cleaved end surfaces of the nitride semiconductorlayer and the stripe-shaped optical waveguide; and

a cleavage guide groove formed, to help form the end surfaces, in thetop surface of the nitride semiconductor layer from above elsewhere thanright above the stripe-shaped optical waveguide.

According to another aspect of the present invention, a method offabricating a nitride semiconductor device as described above includesthe steps of:

adjusting to within the range from 80 to 160 μm the thickness of anitride semiconductor wafer formed by depositing on a substrate thatexhibits cleavage a nitride semiconductor layer formed out of a compoundcontaining a group III element and nitrogen and including a cleavageplane equal to a cleavage plane of the substrate, with a plurality ofstripe-shaped optical waveguides formed at equal intervals in thenitride semiconductor layer;

forming a plurality of cleavage guide grooves in the shape ofdiscontinuous broken lines in the top surface of the nitridesemiconductor wafer by scribing from above the nitride semiconductorlayer in such a way that the cleavage guide grooves reach the substrate;and

cleaving the nitride semiconductor wafer along the cleavage guidegrooves.

Here, the cleavage guide grooves are formed elsewhere than right abovethe stripe-shaped optical waveguides.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other objects and features of the present invention will becomeclear from the following description, taken in conjunction with thepreferred embodiments with reference to the accompanying drawings inwhich:

FIG. 1 is a diagram illustrating the hexagonal crystal structure;

FIG. 2 is a sectional view showing the structure of the wafer before theGaN-based semiconductor laser device of a first embodiment of theinvention is diced apart;

FIG. 3A is a sectional view illustrating the dicing of the wafer in thefirst embodiment;

FIG. 3B is a top view illustrating the dicing of the wafer in the firstembodiment;

FIG. 4 is an external perspective view of the GaN-based semiconductorlaser device of the first embodiment;

FIG. 5 is a sectional view showing the structure of the wafer before theGaN-based semiconductor laser device of a second embodiment of theinvention is diced apart;

FIG. 6A is a sectional view illustrating the dicing of the wafer in thesecond embodiment;

FIG. 6B is a top view illustrating the dicing of the wafer in the secondembodiment;

FIG. 7 is an external perspective view of the GaN-based semiconductorlaser device of the second embodiment;

FIG. 8 is a sectional view showing the structure of the wafer before theGaN-based semiconductor laser device of a third embodiment of theinvention is diced apart;

FIG. 9A is a sectional view illustrating the dicing of the wafer in thethird embodiment;

FIG. 9B is a top view illustrating the dicing of the wafer in the thirdembodiment;

FIG. 10 is an external perspective view of the GaN-based semiconductorlaser device of the third embodiment;

FIG. 11A is a sectional view illustrating the dicing of the wafer in afourth embodiment of the invention;

FIG. 11B is a top view illustrating the dicing of the wafer in thefourth embodiment;

FIG. 12 is an external perspective view of the GaN-based semiconductorlaser device of the fourth embodiment; and

FIG. 13 is a top view illustrating the dicing of a conventional wafer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings. First, the definitions of some terms used inthe present specification will be given.

In the present specification, a “GaN-based semiconductor” denotes aIII-V nitride compound semiconductor that is composed of a group IIIelement such as Al, Ga, or In and a group V element N and that has ahexagonal crystal structure. Examples of GaN-based semiconductorsinclude compounds whose composition is expressed asAl_(X)Ga_(Y)In_(1-X-Y)N (where 0≦X≦1, 0≦Y≦1, and 0≦X+Y≦1), compoundsobtained by substituting another element for part (about 20% or less) ofthe group III element contained in those compounds, and compoundsobtained by substituting another element for part (about 20% or less) ofthe group V element contained in the first-mentioned compounds.

In the present specification, a “GaN-based substrate” denotes, like aGaN-based semiconductor, a substrate formed mainly out of a Ill-Vnitride compound semiconductor that is composed of a group III elementsuch as Al, Ga, or In and a group V element N and that has a hexagonalcrystal structure. Examples of GaN-based substrates include substrateswhose composition is expressed as Al_(X)Ga_(Y)In_(1-X-Y)N (where 0≦X≦1,0≦Y≦1, and 0≦X+Y≦1), substrates obtained by substituting another elementfor part (about 20% or less) of the group III element contained in thosesubstrates, and substrates obtained by substituting another element forpart (about 20% or less) of the group V element contained in thefirst-mentioned substrates. In addition, consider a case where a thicklayer of a GaN-based semiconductor is deposited on a dissimilarsubstrate formed mainly out of a substance other than a GaN-basedsemiconductor, then an LD structure, described later, is laid further ontop to produce a wafer, and eventually the dissimilar substrate isremoved before the wafer is diced in any of the embodiments describedhereinafter. In a case like this, the thick GaN-based semiconductorlayer is to be understood to belong to the category of GaN-basedsubstrates defined in the present specification.

In the present specification, an “LD structure” denotes a structureformed mainly out of a GaN-based semiconductor so as to includelight-emitting portions and waveguide structures before being formedinto cavities. An LD structure is a layer structure deposited, orepitaxially grown, on the GaN-based substrate described above, excludingmetal used as electrodes and insulating film or the like insertedbetween such electrodes and the GaN-based semiconductor. An LD structuremay contain a GaN-based semiconductor having a partially differentcrystal structure or a material other than a GaN-based semiconductor.

In the present specification, a “stripe-shaped optical waveguide”denotes an integral structure, including a light-emitting portion, forconfining and guiding the light emitted from the light-emitting portion.

In the present specification, an “average surface irregularity value Ra”denotes the average value of a surface roughness curve with respect toits center line as measured with a surface roughness tester.

In the present specification, an “RMS value,” or simply “surfaceirregularity average,” denotes the magnitude of surface irregularitiesas obtained by measuring a surface roughness curve over a length of 4 μmparallel to the growth layer surface with an AFM (automatic forcemicroscope) and calculating its RMS (root mean square, the square rootof the sum of the squares of deviations of the surface roughness curvefrom its center line).

In the present specification, a “groove depth” denotes the valueobtained by measuring the depth of a cleavage guide groove or cleavageassist groove vertically from its rim to its bottom with a GaN-basedsemiconductor laser device placed so that the interface between itsGaN-based substrate and LD structure is horizontal. A groove depth maybe of one of the following three types depending on where the rim isconsidered to be located: (1) a groove depth with respect to the surfaceof the GaN-based semiconductor laid as the LD structure; (2) a groovedepth with respect to the center line of the interface between theGaN-based substrate and the LD structure, and (3) a groove depth withrespect to the bottom surface of the GaN-based substrate.

Moreover, it is assumed that, in a hexagonal crystal structure as shownin FIG. 1, <0001> represents all the directions of normals to surfacesidentified as “A,” specifically [0001] and [000-1]; <1-100> representsall the directions of normals to surfaces identified as “B,”specifically [1-100], [10-10], [01-10], [-1100], [-1010], and [0-110];and <11-20> represents all the directions of normals to surfacesidentified as “C,” specifically [11-20], [1210], [-2110], [-1-120],[-12-10], and [2-1-10].

In the following descriptions of the embodiments, GaN-basedsemiconductor laser devices are taken up as representatives of nitridesemiconductor devices.

FIRST EMBODIMENT

A first embodiment of the invention will be described below withreference to the drawings. FIG. 2 is a sectional view showing thestructure of the wafer before the GaN-based semiconductor laser deviceof this embodiment is diced apart. FIGS. 3A and 3B are a sectional viewand a top view, respectively, illustrating the dicing of the wafer. FIG.4 is an external perspective view of the GaN-based semiconductor laserdevice diced apart.

1. Fabrication Processes of the GaN-Based Semiconductor Laser Device(Forming the Wafer)

First, how the wafer is formed will be described with reference to FIG.2.

An n-GaN-based substrate 200 having a crystal growth surface on the(0001) plane and having a film thickness of 100 to 500 μm is cleanedorganically. In this embodiment, the film thickness of the n-GaN-basedsubstrate 200 is adjusted to 135 μm.

The cleaned n-GaN-based substrate is then transported into MOCVD (metalorganic chemical vapor deposition) equipment, and is subjected, in anatmosphere of hydrogen (H₂), to cleaning at a high temperature of about1,100° C. Thereafter, the temperature is lowered, and then, along with aflow of H₂ as a carrier gas flowing at a flow rate of 10 l/min, silane(SiH₄) is introduced at 10 nmol/min, and then, at 600° C., ammonia (NH₃)and trimethylgallium (TMG) are introduced at 5 l/min and 20 mol/min,respectively, to grow an n-GaN-based buffer layer 201 having a thicknessof 10 nm to 10 μm (for example, 100 nm).

This buffer layer 201 may be a GaN buffer layer formed with SiH₄introduced at 0 mol/min, or a GaN buffer layer containing Al or In. Tomake the buffer layer contain Al or In, an adequate amount oftrimethylaluminum (TMA) or trimethylindium (TMI), respectively, isintroduced during film formation. The buffer layer 201 is a layer formedfor the purpose of alleviating the surface distortion of and improving(flattening) the surface morphology and irregularities of then-GaN-based substrate, and therefore may be omitted if, in then-GaN-based substrate, the n-GaN for crystal growth has superbcrystallinity.

Next, with a flow of nitrogen (N₂) and ammonia (NH₃) at 5 l/min each,the temperature is raised up to about 1,050° C. Thereafter, the carriergas is switched from N₂ to H₂, and TMG and SiH₄ are introduced at 100μmol/min and 10 nmol/min, respectively, to grow an n-GaN contact layer202 having a thickness of 0.1 to 10 μm (for example, 4 μm).

Next, the flow rate of TMG is adjusted to 50 μmol/min, and a prescribedamount of TMA is introduced to lay a layer of n-Al_(X1)Ga_(1-X1)N(where, for example, x1=0.2) and thereby form an n-AlGaN clad layer 203having a total film thickness of, for example, 0.8 μm. This n-AlGaN cladlayer 203 may be replaced with a film of another material as long as thematerial has a low refractive index and a large band gap compared withthe n-GaN optical guide layer 204 described later. It is also possibleto combine a number of layers to make them as a whole have an averagerefractive index and an average band gap that fulfill the conditionsstated just above in comparison with the n-GaN optical guide layer 204.

After the formation of the n-AlGaN clad layer 203, the supply of TMA isstopped, and the flow rate of TMG is adjusted to 100 μmol/min to growthe n-GaN optical guide layer 204 until it has a thickness of 50 to 200nm (for example, 100 nm). Thereafter, the supply of TMG is stopped, thecarrier gas is switched from H₂ to N₂, and the temperature is lowereddown to 700° C. Then, TMI is introduced in a prescribed amount and TMGat 15 μmol/min to grow a barrier layer of In_(V)Ga_(1-V)N (where 0<v<1).A predetermined period thereafter, the supply of TMI is increased to aprescribed amount to grow a well layer of In_(W)Ga_(1-W)N (where 0<w<1).

This variation in the amount of TMI supplied is repeated to form anInGaN multiple quantum well active layer 205 having an alternating layerstructure composed of InGaN barrier layers and InGaN well layers. Thecompositions and film thicknesses of the InGaN with which the barrierand well layers are formed are so designed that the light emitted has awavelength in the range from 370 to 430 nm, and the flow rate of TMIintroduced during growth is so adjusted as to obtain films having thedesigned In compositions.

The InGaN multiple quantum well active layer 205 has, preferably, 2 to 6well layers and, particularly preferably, 3 well layers. On completionof the formation of the InGaN multiple quantum well active layer 205,the supply of TMI and TMG is stopped, and the temperature is raised backup to 1,050° C. Then, the carrier gas is switched again from N₂ to H₂,and then, with a flow of TMG at 50 μmol/min, TMA in an adequate amount,and bis(cyclopentadienyl)magnesium (Cp₂Mg), which is a p-type dopingmaterial, at 10 nmol/min, an evaporation prevention layer 206 ofp-Al_(Z)Ga_(1-Z)N (0≦z≦0.3) having a thickness of 0 to 20 nm is grown.On completion of the growth of this p-AlGaN evaporation prevention layer206, the supply of TMA is stopped, and the amount of TMG supplied isadjusted to 100 μmol/min to grow a p-GaN optical guide layer 207 havinga thickness of 50 to 200 nm (for example, 100 nm).

Next, the flow rate of TMG is adjusted to 50 μmol/min, and a prescribedamount of TMA is introduced to lay a p-type Al_(X2)Ga_(1-X2)N layer(where, for example, x2=0.2) and thereby form a p-AlGaN clad layer 208having a total film thickness of, for example, 0.8 μm. This p-AlGaN cladlayer 208 may be replaced with a film of another material as long as thematerial has a low refractive index and a large band gap compared withthe p-GaN optical guide layer 207. It is also possible to combine anumber of layers to make them as a whole have an average refractiveindex and an average band gap that fulfill the conditions stated justabove in comparison with the p-GaN optical guide layer 207.

Lastly, the amount of TMG supplied is adjusted to 100 μmol/min, and thesupply of TMA is stopped to grow a p-GaN contact layer 209 having a filmthickness of 0.01 to 10 μm (for example, 0.1 μm). With this ends thegrowth of an LD structure on top of the GaN-based substrate 200. Oncompletion of the growth, the supply of TMG and Cp₂Mg is stopped and thetemperature is lowered, and then, at room temperature, the wafer istransported out of the MOCVD equipment. With a wafer actually formed inthis way, we measured its surface flatness and observed that the averagesurface irregularity value was Ra=100 Å.

Subsequently, the wafer is subjected to a series of processes to formindividual laser devices. First, in the process of forming p-electrodes,etching is performed in the shape of stripes in the <1-100> direction(see FIGS. 3A and 3B) of the GaN-based substrate 200 to form ridgedstripes 211. Thereafter, a SiO₂ dielectric film 212 is vapor-deposited,then the p-GaN contact layer 209 is exposed, and then Pd, Mo, and Au arevapor-deposited in this order to form p-electrodes 213. The p-electrodes213 may be formed by vapor-depositing Pd, Pt, and Au in this order, orPd and Au in this order, or Ni and Au in this order.

Next, by a physical process such as polishing, or by a chemical processsuch as wet etching or dry etching, the bottom surface of the n-GaNsubstrate 200 is polished to adjust the thickness of the wafer to 80 to160 μm. In this way, the thickness of the wafer is adjusted to make thewafer easy to dice. Specifically, a wafer with a thickness smaller thanthe lower limit of the range makes its handling during device formationdifficult; by contrast, a wafer with a thickness greater than the upperlimit of the range makes its dicing difficult.

Next, from below the bottom surface of the n-GaN substrate 200, Hf andAl are vapor-deposited in this order to form n-electrodes 210. Using Hfin the n-electrodes 210 in this way is effective in reducing the contactresistance of the n electrodes. The n-electrodes 210 may by formed byvapor-depositing Ti and Al in this order, or Ti and Mo in this order, orHf and Au in this order, or any other suitable combination of materials.

In the process of forming the n-electrodes 210, instead of forming themfrom below the bottom surface of the n-GaN substrate 200, they may beformed on the n-GaN layer 202 exposed from above the top surface of thewafer by dry etching.

Dicing the Wafer

Next, how the wafer is diced in this embodiment will be described withreference to FIGS. 3A and 3B. FIGS. 3A and 3B are a sectional view and atop view, respectively, of the wafer having the LD structure formed ontop of the GaN-based substrate 200 as described above.

In FIGS. 3A and 3B, to simplify the explanations, it is assumed that theGaN-based substrate 250 includes the n-GaN-based substrate 200, n-GaNbuffer layer 201, and n-electrodes 210, and that the LD structure 251includes the n-GaN contact layer 202, n-AlGaN clad layer 203, n-GaNoptical guide layer 204, InGaN multiple quantum well active layer 205,p-AlGaN evaporation prevention layer 206, p-GaN optical guide layer 207,p-AlGaN clad layer 208, p-GaN contact layer 209, SiO₂ dielectric film212, and p-electrodes 213.

In the wafer having the LD structure 251 formed on the GaN-basedsubstrate 250 as described above, stripe-shaped waveguides 253 areformed inside the LD structure 251 as shown in FIG. 3B. Thestripe-shaped waveguides 253 are formed parallel to the <1-100>direction. Between every two adjacent ones of the stripe-shapedwaveguides 253, cleavage guide grooves 252 are formed to help cleave thewafer into a plurality of bars in the direction of <11-20>. The waferhas, where the cleavage guide grooves 252 are formed, a section as shownin FIG. 3A.

The cleavage guide grooves 252 are formed, by scribing using a diamondneedle, in the top surface of the LD structure 251 between every twoadjacent ones of the stripe-shaped waveguides 253 as shown in FIG. 3B.Here, the depth d from the top surface of the LD structure 251 to thedeepest end of the cleavage guide grooves 252 is so controlled as to beat least within the range 1 μm≦d≦10 μm. This helps improve the “baryield rate,” i.e., the rate at which bars are obtained without defectivebreakage when the wafer is cleaved into bars.

In a case where the LD structure 251 is thin enough to permit thecleavage guide grooves 252 to reach the interface between the GaN-basedsubstrate 250 and LD structure 251, the depth d1 from the interfacebetween the GaN-based substrate 250 and LD structure 251 to the deepestend of the cleavage guide grooves 252 may be so controlled as to be inthe range 1 μm≦d1≦10 μm. This helps improve the “device yield rate,”i.e., the rate at which the individual devices obtained from the barscleaved apart are such that the end surfaces of the laser cavity are soflat as to have an RMS value of 0.5 nm or less and that variations inthe cavity length are within a prescribed range.

This is because it is thereby possible to prevent degradation of thefar-field pattern (FFP) of the emitted light and lowering of thereflectivity on the end surfaces of the cavity due to unwanted surfaceirregularities resulting from the LD structure 251 partially containinga substance that does not cleave or a substance that cleaves in adifferent direction and thereby permits the dicing pressure to scatteraround.

As described above, the cleavage guide grooves 252 run in the <11-20>direction of the GaN-based substrate 250. By making the cleavage guidegrooves 252 start and end at points 50 μm or more away from thestripe-shaped waveguides 253, it is possible to cleave the wafer with anincreased bar yield rate. Further preferably, the start and end pointsof the cleavage guide grooves 252 are located 100 μm or more away fromthe stripe-shaped waveguides 253 to achieve an increased device yieldrate.

In this embodiment, as shown in FIG. 3A, the depth from the interfacebetween the GaN-based substrate 250 and the LD structure 251 to thedeepest end of the cleavage guide grooves 252 is uniformly 1 μm. On theother hand, the distance from the stripe-shaped waveguides 253 to thestart or end points of the cleavage guide grooves 252 is 125 μm.

For a maximum bar yield rate in the dicing of the wafer, each stroke ofthe cleavage guide grooves 252 is formed, preferably, in the shape of asolid line as long as possible in the <11-20> direction within theregion described above, but may be formed in the shape of a broken line.The cleavage guide grooves 252 may be formed, instead of by scribing asdescribed above, by dry etching such as RIE (reactive ion etching) orwet etching.

Moreover, the cleavage guide grooves 252 may be formed, instead ofbetween every two adjacent ones of the stripe-shaped waveguides 253 asshown in FIG. 3B, so that the interval between every two adjacent onesof the cleavage guide grooves 252 is 1 mm or shorter and simultaneouslythat the distance from their start and end points to the stripe-shapedwaveguides 253 fulfills the condition described above. This eliminatesthe need to form the cleavage guide grooves 252 between every twoadjacent ones of the stripe-shaped waveguides 253.

Next, the wafer, having the cleavage guide grooves 252 formed in thisway, is cleaved in the <11-20> direction into bars. The dicing of thewafer here is achieved by pressing a breaking blade onto the bottomsurface of the GaN-based substrate 250, in the positions where thecleavage guide grooves 252 are located, from below so as to break thewafer. In this way, in the bars cleaved apart, end surfaces are formedby exploiting cleaved surfaces at which the stripe-shaped waveguides 253are split. The dicing of the wafer may be achieved by cleaving, i.e.,hitting the wafer with a blade so that it is broken by the shock, or byheating the portions around the scribed lines locally, or by breakingusing the shock caused by a sound wave or a stream of water or the like.

By cleaving in this way a wafer as shown in FIGS. 3A and 3B, we obtaineda large number of bars with a cavity length of 500 μm. The actuallymeasured cavity lengths were within ±5 μm of the design value of 500 μm,and the bar yield rate and the device yield rate were over 92% and 90%,respectively. Moreover, in the bars thus cleaved apart, we measured theaverage of the surface irregularities on the end surfaces between thecleavage guide grooves 252 and observed that, whereas at distances of 50μm or shorter from the cleavage guide grooves the RMS value was as greatas 10 nm at the maximum, at distances 100 μm or longer from the cleavageguide grooves, the end surfaces were so flat as to show an RMS value of0.5 nm at the maximum. Considering that the RMS value of the endsurfaces formed in a GaN-based semiconductor laid on a sapphiresubstrate is 3.5 nm on average, the results obtained in this embodimentattest to an improvement in quality in terms of the flatness of thecleaved end surfaces.

The bars thus obtained by cleaving a wafer as shown in FIGS. 3A and 3Bare then further cleaved, by performing scribing in their bottom or topsurface in the <1-100> direction between every two adjacentstripe-shaped waveguides 253, into individual GaN-based semiconductorlaser devices. Here, the scribing may be performed with such a stylusforce (the load with which a stylus is pressed onto the wafer) as topress the bars and thereby brake them in the <1-100> direction intoGaN-based semiconductor laser devices, or may be performed in such a wayas to completely cut the bars into GaN-based semiconductor laserdevices.

2. Structure of the GaN-Based Semiconductor Laser Device

With reference to FIG. 4, the structure of the semiconductor laserdevice 1 formed by being diced apart from a wafer as described abovewill be described.

In FIG. 4, to simplify the explanations, it is assumed that theGaN-based substrate 10 includes the n-GaN-based substrate 200 and n-GaNbuffer layer 201, and that the LD structure 11 includes the n-GaNcontact layer 202, n-AlGaN clad layer 203, n-GaN optical guide layer204, InGaN multiple quantum well active layer 205, p-AlGaN evaporationprevention layer 206, p-GaN optical guide layer 207, p-AlGaN clad layer208, p-GaN contact layer 209, and SiO₂ dielectric film 212.

In the GaN-based semiconductor laser device 1 obtained by dicing a waferhaving an LD structure formed on a GaN-based substrate as describedabove, mirror end surfaces 12 are formed by cleavage on the LD structure11 formed on the GaN-based substrate 10. Inside the LD structure 11, astripe-shaped waveguide 13 is provided that serves to guide laser light.

An n-electrode 210 is formed on the bottom surface of the GaN-basedsubstrate 10, and a p-electrode 213 is formed on the top surface of theLD structure 11. To these electrodes, electric power is supplied fromoutside to operate the GaN-based semiconductor laser device 1. In thefour corners in the top surface of the GaN-based semiconductor laserdevice 1, on its LD structure 11 side, cutaway portions 14 are formed.

The cutaway portions 14 correspond to the cleavage guide grooves 252(see FIGS. 3A and 3B) that have been formed in the top surface of thewafer beforehand to help produce the mirror end surfaces 12 when thewafer is cleaved into bars. In this embodiment, the depth from theinterface between the GaN-based substrate 10 and the LD structure 11 tothe deepest end of the cutaway portions 14 is 1 μm. Moreover, when theGaN-based semiconductor laser device 1 is viewed in a two-dimensionalprojection with the GaN-based substrate 10 down, the cutaway portions 14are so formed as to start at 125 μm away from the stripe-shapedwaveguide 13. In this embodiment, the GaN-based semiconductor laserdevice 1 has four cutaway portions 14. In practice, however, the numberof cutaway portions 14 formed in the GaN-based semiconductor laserdevice 1 may vary, starting from at least one, depending on how thecleavage guide grooves 252 are formed in the top surface of the waferbeforehand.

In this embodiment, when the wafer is diced to form the GaN-basedsemiconductor laser device, the cutaway portions that are formed asremnants of the cleavage guide grooves may be cut off altogether. Thishas the advantage of removing the dust or the like produced when thecleavage guide grooves are formed.

SECOND EMBODIMENT

A second embodiment of the invention will be described below withreference to the drawings. FIG. 5 is a sectional view showing thestructure of the wafer before the GaN-based semiconductor laser deviceof this embodiment is diced apart. FIGS. 6A and 6B are a sectional viewand a top view, respectively, illustrating the dicing of the wafer. FIG.7 is an external perspective view of the GaN-based semiconductor laserdevice diced apart.

1. Fabrication Processes of the GaN-Based Semiconductor Laser Device(Forming the Wafer)

First, how the wafer is formed will be described with reference to FIG.5. In FIG. 5, such elements as are found also in the wafer shown in FIG.2 are identified with the same reference numerals, and their detaileddescriptions will not be repeated.

In this embodiment, unlike the first embodiment (FIG. 2), first, on thetop surface of an n-GaN-based substrate 200 having a crystal growthsurface on the (0001) plane and having a film thickness of 100 to 500μm, SiO₂ is vapor-deposited by a process using an electron beam or bysputtering to form a growth suppression film. Thereafter, by the use ofa lithography technique using a photo-curing resin, stripe-shaped SiO₂masks 501 are formed, from the growth suppression film formed on then-GaN-based substrate 200, along the <1-100> direction of then-GaN-based substrate 200.

The masks 501 have a mask width of 13 μm each and are arranged with 7 μmwide windows secured between them. The growth suppression film may beformed out of any other material than SiO₂, for example, SiN_(X), Al₂O₃,or TiO₂. The masks may be hollow cavities.

The n-GaN-based substrate 200 having the masks 501 formed on its topsurface in this way is then cleaned organically, and is then transportedinto MOCVD equipment, where, along with a flow of H₂ as a carrier gas,TMG is introduced at 100 μmol/min and SiH₄ at 10 nmol/min to grow ann-GaN contact layer 502 having a thickness of 25 μm.

After this n-GaN contact layer 502 is formed, as in the firstembodiment, TMA is introduced to form an n-AIGaN clad layer 203, andthen the supply of TMA is stopped to form an n-GaN optical guide layer204. Thereafter, the carrier gas is switched from H₂ back to N₂ and thetemperature is lowered down to 700° C., and then TMI and TMG areintroduced to form an InGaN multiple quantum well active layer 205having an alternating layer structure composed of InGaN barrier layersand InGaN well layers.

Then, the supply of TMI and TMG is stopped, the temperature is raisedback up to 1,050° C., the carrier gas is switched from N₂ back to H₂,and then TMG, TMA, and Cp₂Mg are introduced to grow a p-AlGaNevaporation prevention layer 206. Thereafter, the supply of TMA isstopped to grow a p-GaN optical guide layer 207. Furthermore, TMA isintroduced again to form a p-AlGaN clad layer 208. Finally, the supplyof TMA is stopped to grow an p-GaN contact layer 209. With this ends thegrowth of an LD structure on top of the n-GaN-based substrate 200.

Then, the supply of TMG and Cp₂Mg is stopped and the temperature islowered, and then the wafer having the LD structure formed on its topsurface is transported out of the MOCVD equipment. The wafer is thensubjected to a series of processes to form individual laser devices.First, etching is performed to form ridged stripes 211, then a SiO₂dielectric film 212 is vapor-deposited, then the p-GaN contact layer 209is exposed, and then Pd, Mo, and Au are vapor-deposited in this order toform p-electrodes 213. As in the first embodiment, the p-electrodes 213may be formed by vapor-depositing Pd, Pt, and Au in this order, or Pdand Au in this order, or Ni and Au in this order.

Next, by a physical process such as polishing, or by a chemical processsuch as wet etching or dry etching, the bottom surface of the n-GaNsubstrate 200 is polished to adjust the thickness of the wafer to 80 to160 μm. Next, from below the bottom surface of the n-GaN substrate 200,Hf and Al are vapor-deposited in this order to form n-electrodes 210. Asin the first embodiment, the n-electrodes 210 may by formed byvapor-depositing Ti and Al in this order, or Ti and Mo in this order, orHf and Au in this order, or any other suitable combination of materials.

In the process of forming the n-electrodes 210, instead of forming themfrom below the bottom surface of the n-GaN substrate 200, they may beformed on the n-GaN layer 202 exposed from above the top surface of theepitaxial wafer by dry etching.

Dicing the Wafer

Next, how the wafer is diced in this embodiment will be described withreference to FIGS. 6A and 6B. FIGS. 6A and 6B are a sectional view and atop view, respectively, of the wafer having the LD structure formed ontop of the GaN-based substrate 200 as described above.

In FIGS. 6A and 6B, to simplify the explanations, it is assumed that theGaN-based substrate 510 includes the n-GaN-based substrate 200 andn-electrodes 210, and that the LD structure 511 includes the mask 501,n-GaN contact layer 502, n-AIGaN clad layer 203, n-GaN optical guidelayer 204, InGaN multiple quantum well active layer 205, p-AlGaNevaporation prevention layer 206, p-GaN optical guide layer 207, p-AlGaNclad layer 208, p-GaN contact layer 209, SiO₂ dielectric film 212, andp-electrodes 213.

In the wafer having the LD structure 511 formed on the GaN-basedsubstrate 510 as described above, just as in the first embodiment,stripe-shaped waveguides 253 are formed inside the LD structure 511 asshown in FIG. 6B. Between every two adjacent ones of the stripe-shapedwaveguides 253, cleavage assist grooves 512 and cleavage guide grooves513 are formed to help cleave the wafer into a plurality of bars in thedirection of <11-20>. The wafer has, where the cleavage assist grooves512 and the cleavage guide grooves 513 are formed, a section as shown inFIG. 6A.

First, the cleavage assist grooves 512 are formed in the surface of theLD structure 511 by performing RIE between the stripe-shaped waveguides253 as shown in FIG. 6B. The cleavage assist grooves 512 are so formedas to have a depth half the thickness of the epitaxial layer of the LDstructure 511. That is, the cleavage assist grooves 512 are formed abovethe masks 501 formed inside the LD structure 511.

After the cleavage assist grooves 512 are formed in this way, thecleavage guide grooves 513 are formed, by scribing using a diamondneedle, substantially along the center line of the cleavage assistgrooves 512. The cleavage guide grooves 513 are formed so as to reachthe masks 501 formed inside the LD structure 511, which has a differentcleavage plane from the other part of the LD structure 511. Here, thedepth d2 from the top surface of the LD structure 511 to the deepest endof the cleavage guide grooves 513 is so controlled as to fulfill 1μm≦d2≦10 μm. This helps improve the bar yield rate.

In a case where the LD structure 511 is thin enough to permit thecleavage guide grooves 513 to reach the interface between the GaN-basedsubstrate 510 and LD structure 511, the depth d3 from the interfacebetween the GaN-based substrate 510 and LD structure 511 to the deepestend of the cleavage guide grooves 513 may be so controlled as to be inthe range 1μm≦d3≦10 μm. This helps improve the device yield rate.

This is because it is thereby possible to prevent degradation of the FFPof the emitted light and lowering of the reflectivity on the endsurfaces of the cavity due to unwanted surface irregularities resultingfrom the LD structure 551 partially containing a substance that does notcleave or a substance that cleaves in a different direction and therebypermits the dicing pressure to scatter around, for example the masks 501in particular.

As described above, the cleavage assist grooves 512 and the cleavageguide grooves 513 run in the <11-20> direction of the GaN-basedsubstrate 510. By making the cleavage assist grooves 512 and thecleavage guide grooves 513 start and end at points 50 μm or more awayfrom the stripe-shaped waveguides 253, it is possible to cleave thewafer with an increased bar yield rate. Further preferably, the startand end points of the cleavage assist grooves 512 and the cleavage guidegrooves 513 are located 100 μm or more away from the stripe-shapedwaveguides 253 to achieve an increased device yield rate.

In this embodiment, as shown in FIG. 6A, the depth from the interfacebetween the GaN-based substrate 510 and the LD structure 511 to thedeepest end of the cleavage guide grooves 513 is uniformly 1 μm. On theother hand, the distance from the stripe-shaped waveguides 253 to thestart or end points of the cleavage assist grooves 512 and the cleavageguide grooves 513 is 125 μm.

For a maximum bar yield rate in the dicing of the wafer, each stroke ofthe cleavage assist grooves 512 and the cleavage guide grooves 513 isformed, preferably, in the shape of a solid line as long as possible inthe <11-20> direction within the region described above, but may beformed in the shape of a broken line. The cleavage guide grooves 513 maybe formed, instead of by scribing as described above, by dry etchingsuch as RIE or wet etching.

Moreover, the cleavage assist grooves 512 and the cleavage guide grooves513 may be formed, instead of between every two adjacent ones of thestripe-shaped waveguides 253 as shown in FIG. 6B, so that the intervalbetween every two adjacent ones of the cleavage assist grooves 512 andthe cleavage guide grooves 513 is 1 mm or shorter and simultaneouslythat the distance from their start and end points to the stripe-shapedwaveguides 253 fulfills the condition described above. This eliminatesthe need to form the cleavage assist grooves 512 and the cleavage guidegrooves 513 between every two adjacent ones of the stripe-shapedwaveguides 253.

Next, the wafer, having the cleavage assist grooves 512 and the cleavageguide grooves 513 formed in this way, is cleaved in the <11-20>direction into bars. The dicing of the wafer here is achieved, as in thefirst embodiment, by pressing a breaking blade onto the bottom surfaceof the GaN-based substrate 510, in the positions where the cleavageguide grooves 513 are located, from below so as to break the wafer. Inthis way, in the bars cleaved apart, end surfaces are formed byexploiting cleaved surfaces at which the stripe-shaped waveguides 253are split. The dicing of the wafer may be achieved by cleaving, i.e.,hitting the wafer with a blade so that it is broken by the shock, or byheating the portions around the scribed lines locally, or by breakingusing the shock caused by a sound wave or a stream of water or the like.

By cleaving in this way a wafer as shown in FIGS. 6A and 6B, we obtaineda large number of bars with a cavity length of 500 μm. The actuallymeasured cavity lengths were within ±5 μm of the design value of 500 μm,and the bar yield rate and the device yield rate were over 92% and 96%,respectively. Moreover, in the bars thus cleaved apart, we measured theaverage of the surface irregularities on the end surfaces between thecleavage assist grooves 512 and obtained results comparable to thoseobtained in the first embodiment. This attests to an improvement inquality in terms of the flatness of the cleaved end surfaces comparedwith a case where a GaN-based semiconductor is laid on a sapphiresubstrate.

The bars thus obtained by cleaving a wafer as shown in FIGS. 6A and 6Bare then, as in the first embodiment, further cleaved, by performingscribing in their bottom or top surface in the <1-100> direction betweenevery two adjacent stripe-shaped waveguides 253, into individualGaN-based semiconductor laser devices. Here, the scribing may beperformed with such a stylus force as to press the bars and therebybrake them in the <1-100> direction into GaN-based semiconductor laserdevices, or may be performed in such a way as to completely cut the barsinto GaN-based semiconductor laser devices.

2. Structure of the GaN-Based Semiconductor Laser Device

With reference to FIG. 7, the structure of the semiconductor laserdevice 2 formed by being diced apart from a wafer as described abovewill be described.

In FIG. 7, to simplify the explanations, it is assumed that the LDstructure 21 includes the masks 501, n-GaN contact layer 502, n-AlGaNclad layer 203, n-GaN optical guide layer 204, InGaN multiple quantumwell active layer 205, p-AlGaN evaporation prevention layer 206, p-GaNoptical guide layer 207, p-AlGaN clad layer 208, p-GaN contact layer209, and SiO₂ dielectric film 212.

In the GaN-based semiconductor laser device 2 obtained by dicing a waferhaving an LD structure formed on a GaN-based substrate as describedabove, mirror end surfaces 22 are formed by cleavage on the LD structure21 formed on the GaN-based substrate 200. Inside the LD structure 21, astripe-shaped waveguide 23 is provided that serves to guide laser light.

An-electrode 210 is formed on the bottom surface of the GaN-basedsubstrate 200, and a p-electrode 213 is formed on the top surface of theLD structure 21. To these electrodes, electric power is supplied fromoutside to operate the GaN-based semiconductor laser device 2. In thefour corners in the top surface of the GaN-based semiconductor laserdevice 2, on its LD structure 21 side, cutaway portions 24 are formed.The cutaway portions 24 are each composed of a portion 24 a having adepth reaching about the middle of the LD structure 21 and a portion 24b extending from the bottom surface of the portion 24 a and reaching theGaN-based substrate 200.

The cutaway portions 24 correspond to the cleavage assist grooves 512and the cleavage guide grooves 513 (see FIGS. 6A and 6B) that have beenformed in the top surface of the wafer beforehand to help produce themirror end surfaces 22 when the wafer is cleaved into bars, with theportions 24 a corresponding to the cleavage assist grooves 512 and theportions 24 b to cleavage guide grooves 513. In this embodiment, thedepth from the interface between the GaN-based substrate 200 and the LDstructure 21 to the deepest end of the cutaway portions 24 is 1 μm.

Moreover, when the GaN-based semiconductor laser device 2 is viewed in atwo-dimensional projection with the GaN-based substrate 200 down, thecutaway portions 24 are so formed as to start at 125 μm away from thestripe-shaped waveguide 23. In this embodiment, the GaN-basedsemiconductor laser device 2 has four cutaway portions 24. In practice,however, the number of cutaway portions 24 formed in the GaN-basedsemiconductor laser device 2 may vary, starting from at least one,depending on how the cleavage assist grooves 512 and the cleavage guidegrooves 513 are formed in the top surface of the wafer beforehand.

In this embodiment, when the wafer is diced to form the GaN-basedsemiconductor laser device, the cutaway portions that are formed asremnants of the cleavage guide grooves may be cut off altogether. Thishas the advantage of removing the dust or the like produced when thecleavage guide grooves are formed.

THIRD EMBODIMENT

A third embodiment of the invention will be described below withreference to the drawings. FIG. 8 is a sectional view showing thestructure of the wafer before the GaN-based semiconductor laser deviceof this embodiment is diced apart. FIGS. 9A and 9B are a sectional viewand a top view, respectively, illustrating the dicing of the wafer. FIG.10 is an external perspective view of the GaN-based semiconductor laserdevice diced apart.

1. Fabrication Processes of the GaN-Based Semiconductor Laser Device(Forming the Wafer)

First, how the wafer is formed will be described with reference to FIG.8. In FIG. 8, such elements as are found also in the wafer shown in FIG.2 are identified with the same reference numerals, and their detaileddescriptions will not be repeated.

In this embodiment, as in the first embodiment (FIG. 2), first, ann-GaN-based substrate 200 having a crystal growth surface on the (0001)plane and having a film thickness of 100 to 500 μm is cleanedorganically, and is then transported into MOCVD equipment, where, in anatmosphere of H₂, cleaning is performed at a high temperature of about1,100° C. Then, in this embodiment, unlike the first embodiment, with N₂and NH₃ flowing at 5 l/min each, the temperature is lowered down toabout 1,050° C., then the carrier gas is switched from N₂ to H₂, andthen TMG and SiH₄ are introduced to grow an n-GaN contact layer 202having a thickness of 0.1 to 10 μm (for example, 4 μm).

Thereafter, an LD structure and n- and p-electrodes are formed in thesame manner as in the first and second embodiments. Specifically, on then-GaN contact layer 202, an n-AlGaN clad layer 203, an N-GaN opticalguide layer 204, an InGaN multiple quantum well active layer 205, ap-AlGaN evaporation prevention layer 206, a p-GaN optical guide layer207, a p-AlGaN clad layer 208, and a p-GaN contact layer 209 are formedin this order to complete the growth of an LD structure. Then, at roomtemperature, the wafer complete with the LD structure is transported outof the MOCVD equipment.

Then, with this wafer complete with the LD structure, etching isperformed on its surface in the shape of stripes to form ridged stripes211, then, a SiO₂ dielectric film 212 is vapor-deposited, and then thep-GaN contact layer 209 is exposed. Further on top, a material forp-electrodes is vapor-deposited to form p-electrodes 213. Moreover, thebottom surface of the n-GaN-based substrate 200 is polished to adjustthe thickness of the wafer, and then a material for n-electrodes isvapor-deposited on the bottom surface of the n-GaN substrate to formn-electrodes 210.

Dicing the Wafer

Next, how the wafer is diced in this embodiment will be described withreference to FIGS. 9A and 9B. FIGS. 9A and 9B are a sectional view and atop view, respectively, of the wafer having the LD structure formed ontop of the GaN-based substrate 200 as described above.

In FIGS. 9A and 9B, to simplify the explanations, it is assumed that theGaN-based substrate 700 includes the n-GaN-based substrate 200 andn-electrodes 210, and that the LD structure 701 includes the n-GaNcontact layer 202, n-AlGaN clad layer 203, n-GaN optical guide layer204, InGaN multiple quantum well active layer 205, p-AlGaN evaporationprevention layer 206, p-GaN optical guide layer 207, p-AlGaN clad layer208, p-GaN contact layer 209, SiO₂ dielectric film 212, and p-electrodes213.

In the wafer having the LD structure 701 formed on the GaN-basedsubstrate 700 as described above, just as in the first embodiment,stripe-shaped waveguides 703 are formed inside the LD structure 701 asshown in FIG. 9B. Between every two adjacent ones of the stripe-shapedwaveguides 703, cleavage guide grooves 702 are formed to help cleave thewafer into a plurality of bars in the direction of <11-20>. The waferhas, where the cleavage guide grooves 702 are formed, a section as shownin FIG. 9A.

The cleavage guide grooves 702 are formed, by scribing using a diamondneedle, in the bottom surface of the GaN-based substrate 700. Here, thedepth d4 from the bottom surface of the GaN-based substrate 700 to thedeepest end of the cleavage guide grooves 702 is so controlled as to beat least within the range 1 μm≦d4≦10 μm. As described above, thecleavage guide grooves 702 are formed in the <11-20> direction of theGaN-based substrate 700. Moreover, by making the cleavage guide grooves702 start and end at 50 μm or more away from the stripe-shapedwaveguides 703, it is possible to cleave the wafer with an increased baryield rate. Further preferably, the start and end points of the cleavageguide grooves 702 are located 100 μm or more away from the stripe-shapedwaveguides 703 to achieve an increased device yield rate.

In this embodiment, as shown in FIG. 9A, the depth d4 from the bottomsurface of the GaN-based substrate 700 to the deepest end of thecleavage guide grooves 702 is uniformly 4 μm. On the other hand, thedistance from the stripe-shaped waveguides 703 to the start or endpoints of the cleavage guide grooves 702 is 125 μm. For a maximum baryield rate in the dicing of the wafer, each stroke of the cleavage guidegrooves 702 is formed, preferably, in the shape of a solid line as longas possible in the <11-20> direction within the region described above,but may be formed in the shape of a broken line. The cleavage guidegrooves 702 may be formed, instead of by scribing as described above, bydry etching such as RIE or wet etching.

Moreover, the cleavage guide grooves 702 may be formed, instead ofbetween every two adjacent ones of the stripe-shaped waveguides 703 asshown in FIG. 9B, so that the interval between every two adjacent onesof the cleavage guide grooves 702 is 1 mm or shorter and simultaneouslythat the distance from their start and end points to the stripe-shapedwaveguides 703 fulfills the condition described above. This eliminatesthe need to form the cleavage guide grooves 702 between every twoadjacent ones of the stripe-shaped waveguides 703.

Next, the wafer, having the cleavage guide grooves 702 formed in thisway, is cleaved in the <11-20> direction into bars. In this embodiment,unlike the first embodiment, the dicing of the wafer is achieved bypressing a breaking blade onto the top surface of the LD structure 701,in the positions where the cleavage guide grooves 702 are located, fromabove so as to break the wafer. In this way, in the bars cleaved apart,end surfaces are formed by exploiting cleaved surfaces at which thestripe-shaped waveguides 703 are split. The dicing of the wafer may beachieved by cleaving, i.e., hitting the wafer with a blade so that it isbroken by the shock, or by heating the portions around the scribed lineslocally, or by breaking using the shock caused by a sound wave or astream of water or the like.

By cleaving in this way a wafer as shown in FIGS. 9A and 9B, we obtaineda large number of bars with a cavity length of 500 μm. The actuallymeasured cavity lengths were within ±5 μm of the design value of 500 μm,and the bar yield rate was over 92%. Moreover, in the bars thus cleavedapart, we measured the average of the surface irregularities on the endsurfaces between the cleavage guide grooves 702 and obtained resultscomparable to those obtained in the first embodiment. This attests to animprovement in quality in terms of the flatness of the cleaved endsurfaces compared with a case where a GaN-based semiconductor is laid ona sapphire substrate.

The bars thus obtained by cleaving a wafer as shown in FIGS. 9A and 9Bare then, as in the first embodiment, further cleaved, by performingscribing in their bottom or top surface in the <1-100> direction betweenevery two adjacent stripe-shaped waveguides 703, into individualGaN-based semiconductor laser devices. Here, the scribing may beperformed with such a stylus force as to press the bars and therebybrake them in the <1-100> direction into GaN-based semiconductor laserdevices, or may be performed in such a way as to completely cut the barsinto GaN-based semiconductor laser devices.

2. Structure of the GaN-Based Semiconductor Laser Device

With reference to FIG. 10, the structure of the semiconductor laserdevice 3 formed by being diced apart from a wafer as described abovewill be described.

In FIG. 10, to simplify the explanations, it is assumed that the LDstructure 31 includes the n-GaN contact layer 202, n-AlGaN clad layer203, n-GaN optical guide layer 204, InGaN multiple quantum well activelayer 205, p-AIGaN evaporation prevention layer 206, p-GaN optical guidelayer 207, p-AlGaN clad layer 208, p-GaN contact layer 209, and SiO₂dielectric film 212.

In the GaN-based semiconductor laser device 3 obtained by dicing a waferhaving an LD structure formed on a GaN-based substrate as describedabove, mirror end surfaces 32 are formed by cleavage on the LD structure31 formed on the GaN-based substrate GaN 200. Inside the LD structure31, a stripe-shaped waveguide 33 is provided that serves to guide laserlight.

An n-electrode 210 is formed on the bottom surface of the GaN-basedsubstrate 200, and a p-electrode 213 is formed on the top surface of theLD structure 31. To these electrodes, electric power is supplied fromoutside to operate the GaN-based semiconductor laser device 3. In thefour corners in the bottom surface of the GaN-based semiconductor laserdevice 3, on its GaN-based substrate 200 side, cutaway portions 34 areformed.

The cutaway portions 34 correspond to the cleavage guide grooves 702(see FIGS. 9A and 9B) that have been formed in the bottom surface of thewafer beforehand to help produce the mirror end surfaces 32 when thewafer is cleaved into bars. In this embodiment, the depth from thebottom surface of the GaN-based substrate 200 to the deepest end of thecutaway portions 34 fulfills 1 μm≦d4≦10 μm.

Moreover, when the GaN-based semiconductor laser device 3 is viewed in atwo-dimensional projection with the GaN-based substrate 200 down, thecutaway portions 34 are so formed as to start at 100 μm away from thestripe-shaped waveguide 33. In this embodiment, the GaN-basedsemiconductor laser device 3 has four cutaway portions 34. In practice,however, the number of cutaway portions 34 formed in the GaN-basedsemiconductor laser device 3 may vary, starting from at least one,depending on how the cleavage guide grooves 702 are formed in the bottomsurface of the wafer beforehand.

In this embodiment, when the wafer is diced to form the GaN-basedsemiconductor laser device, the cutaway portions that are formed asremnants of the cleavage guide grooves may be cut off altogether. Thishas the advantage of removing the dust or the like produced when thecleavage guide grooves are formed.

FOURTH EMBODIMENT

A fourth embodiment of the invention will be described below withreference to the drawings. FIGS. 11A and 11B are a sectional view and atop view, respectively, illustrating the dicing of the wafer. FIG. 12 isan external perspective view of the GaN-based semiconductor laser devicediced apart.

1. Fabrication Processes of the GaN-Based Semiconductor Laser Device(Forming the Wafer)

Here, the wafer formed in this embodiment is assumed to have the samesectional structure as that formed in the first embodiment and shown inthe sectional view in FIG. 2. Therefore, as to how it is formed, thedescription of the first embodiment is to be consulted, and no detailedexplanations will be repeated.

Specifically, on an n-GaN-based substrate 200, an LD structure is formedby growing an n-GaN buffer layer 201, an n-GaN contact layer 202, ann-AlGaN clad layer 203, an n-GaN optical guide layer 204, an InGaNmultiple quantum well active layer 205, a p-AlGaN evaporation preventionlayer 206, a p-GaN optical guide layer 207, a p-AlGaN clad layer 208,and a p-GaN contact layer 209 in this order. Moreover, on the topsurface of the LD structure, ridged stripes 211 are formed, then a SiO₂dielectric film 212 is vapor-deposited, and then the p-GaN contact layer209 is exposed. Further on top, a material for p-electrodes isvapor-deposited to form p-electrodes 213. In addition, the bottomsurface of the n-GaN-based substrate 200 is polished to adjust thethickness of the wafer, and then a material for n-electrodes isvapor-deposited on the bottom surface of the n-GaN-based substrate 200to form n-electrodes 210.

Dicing the Wafer

Next, how the wafer is diced in this embodiment will be described withreference to FIGS. 11A and 11B. FIGS. 11A and 11B are a sectional viewand a top view, respectively, of the wafer having the LD structureformed on top of the GaN-based substrate 200 as described above.

In FIGS. 11A and 11B, as in FIGS. 3A and 3B, to simplify theexplanations, it is assumed that the GaN-based substrate 250 includesthe n-GaN-based substrate 200, n-GaN buffer layer 201, and n-electrodes210, and that the LD structure 251 includes the n-GaN contact layer 202,n-AlGaN clad layer 203, n-GaN optical guide layer 204, InGaN multiplequantum well active layer 205, p-AlGaN evaporation prevention layer 206,p-GaN optical guide layer 207, p-AlGaN clad layer 208, p-GaN contactlayer 209, SiO₂ dielectric film 212, and p-electrodes 213.

Moreover, as in the first embodiment, when the LD structure 251 isformed on the GaN-based substrate 250 to form the wafer, stripe-shapedwaveguides 253 are formed inside the LD structure 251 as shown in FIG.11B.

The wafer formed in this way is placed, with the bottom surface of theGaN-based substrate 250 up, on a dicer provided with a diamond blade,which is a machine for forming grooves in a semiconductor wafer. Then,cleavage assist grooves 254 having a depth d5 (where 0≦d5≦40 μm) and awidth w (0<w≦30 μm) are formed in the shape of broken lines in the<11-20> direction as shown in FIG. 11B. Then, the wafer is turned upsidedown, and, as in the first embodiment, cleavage guide grooves 252 areformed in the shape of solid lines in the top surface of the LDstructure 251 as shown in FIG. 11B by scribing with a diamond needlebetween every two adjacent ones of the stripe-shaped waveguides 253.Thus, whereas the cleavage guide grooves 252 are discontinuous, thecleavage assist grooves 254 are continuous.

Here, when the cleavage guide grooves 252 are formed, the depth d fromthe top surface of the LD structure 251 to the deepest end of thecleavage guide grooves 252 is so controlled as to be at least within therange 1 μm≦d≦10 μm. This helps improve the bar yield rate. Moreover, bycontrolling the depth d1 from the interface between the GaN-basedsubstrate 250 and the LD structure 251 to the deepest end of thecleavage guide grooves 252 within the range 1 μm≦d1≦10 μm, it ispossible to improve the device yield rate.

As described above, the cleavage guide grooves 252 are formed in the<11-20> direction of the GaN-based substrate 250 and in additionsubstantially along the center lines of the cleavage assist grooves 254.By making the cleavage guide grooves 252 start and end at points 50 μmor more away from the stripe-shaped waveguides 253, it is possible tocleave the wafer with an increased bar yield rate. Further preferably,the start and end points of the cleavage guide grooves 252 are located100 μm or more away from the stripe-shaped waveguides 253 to achieve anincreased device yield rate.

In this embodiment, as shown in FIG. 11A, the depth from the interfacebetween the GaN-based substrate 250 and the LD structure 251 to thedeepest end of the cleavage guide grooves 252 is uniformly 1 μm, and thedistance from the stripe-shaped waveguides 253 to the start or endpoints of the cleavage guide grooves 252 is 125 μm. On the other hand,the cleavage assist grooves 254 has a depth d5 of 20 μm and a line widthw of 20 μm, and are arranged with a pitch p of 500 μm between themselvesin the <1-100> direction.

For a maximum bar yield rate in the dicing of the wafer, each stroke ofthe cleavage guide grooves 252 is formed, preferably, in the shape of asolid line as long as possible in the <11-20> direction within theregion described above, but may be formed in the shape of a broken line.The cleavage guide grooves 252 may be formed, instead of by scribing asdescribed above, by dry etching such as RIE or wet etching.

Next, the wafer, having the cleavage guide grooves 252 formed in thisway, is cleaved in the <11-20> direction into bars. The dicing of thewafer here is achieved by pressing a breaking blade onto the cleavageassist grooves 254 from below the bottom surface of the GaN-basedsubstrate 250, in the positions where the cleavage guide grooves 252 arelocated, from below so as to break the wafer. In this way, in the barscleaved apart, end surfaces are formed by exploiting cleaved surfaces atwhich the stripe-shaped waveguides 253 are split. The dicing of thewafer may be achieved by cleaving, i.e., hitting the wafer with a bladeso that it is broken by the shock, or by heating the portions around thescribed lines locally, or by breaking using the shock caused by a soundwave or a stream of water or the like.

By cleaving in this way a wafer as shown in FIGS. 11A and 11B, weobtained a large number of bars with a cavity length of 500 μm. Theactually measured cavity lengths were within ±5 μm of the design valueof 500 μm, and the bar yield rate was over 96%. Moreover, in the barsthus cleaved apart, we measured the average of the surfaceirregularities on the end surfaces between the cleavage guide grooves252 and obtained results comparable to those obtained in the firstembodiment. This attests to an improvement in quality in terms of theflatness of the cleaved end surfaces compared with a case where aGaN-based semiconductor is laid on a sapphire substrate.

The bars thus obtained by cleaving a wafer as shown in FIGS. 11A and 11Bare then further cleaved, by performing scribing in their bottom or topsurface in the <1-100> direction between every two adjacentstripe-shaped waveguides 253, into individual GaN-based semiconductorlaser devices. Here, the scribing may be performed with such a stylusforce as to press the bars and thereby brake them in the <1-100>direction into GaN-based semiconductor laser devices, or may beperformed in such a way as to completely cut the bars into GaN-basedsemiconductor laser devices.

2. Structure of the GaN-Based Semiconductor Laser Device

With reference to FIG. 12, the structure of the semiconductor laserdevice 1 a formed by being diced apart from a wafer as described abovewill be described. In the semiconductor laser device 1 a shown in FIG.12, such elements as are found also in the GaN-based semiconductor laserdevice 1 shown in FIG. 4 are identified with the same referencenumerals, and their detailed explanations will not be repeated.

In the GaN-based semiconductor laser device 1 a fabricated by thefabrication method of this embodiment, as in the first embodiment, inthe four corners in its top surface, on its LD structure 11 side,cutaway portions 14 are formed, which correspond to the cleavage guidegrooves 252 (FIGS. 11A and 11B). Moreover, in the bottom surface of theGaN-based substrate 10, in two places on the mirror end surface side,cutaway portions 15 are formed. These cutaway portions 15 correspond tothe cleavage assist grooves 254 (FIGS. 11A and 11B) that have beenformed in the bottom surface of the wafer beforehand.

Although the GaN-based semiconductor laser device 1 a has four cutawayportions 14 in this embodiment, the number of cutaway portions 14 formedin practice may vary, starting from at least one, depending on how thecleavage guide grooves 252 are formed in the top surface of the waferbeforehand. Likewise, although two cutaway portions 15 are formed inthis embodiment, the number of cutaway portions 15 formed in practicemay vary, starting from at least one, depending on how the cleavageassist grooves 254 are formed in the bottom surface of the waferbeforehand.

In this embodiment, when the wafer is diced to form the GaN-basedsemiconductor laser device, the cutaway portions that are formed asremnants of the cleavage guide grooves may be cut off altogether. Thishas the advantage of removing the dust or the like produced when thecleavage guide grooves are formed.

In this embodiment, a wafer having a structure similar to that of thefirst embodiment is diced into individual GaN-based semiconductor laserdevices by forming cleavage guide grooves and cleavage assist grooves inthe top and bottom surfaces, respectively, of the wafer. Instead, it isalso possible to dice a wafer having a structure similar to that of thesecond embodiment (FIG. 5) into individual GaN-based semiconductor laserdevices by forming cleavage guide grooves and cleavage assist grooves inthe top and bottom surfaces, respectively, of the wafer.

In all the embodiments described above, a particular plane is selectedas the direction in which to form mirror end surfaces. However, it isalso possible to select instead a plane parallel to any of the {0001},{11-20}, and {1-100} planes, which are cleavage planes inherent in aGaN-based semiconductor having a hexagonal crystal structure. Of theseplanes, the {1-100} plane is preferred because it shows good cleavage.Specifically, it is preferable to form mirror end surfaces on one of the(1-100), (10-10), (01-10), (-1100), (-1010), and (0-110) planes.

It is to be understood that the present invention applies not only tosemiconductor laser devices having optical waveguide structures asspecifically described in the embodiments above. That is, the presentinvention applies not only to the ridge structure dealt with in theembodiments above but also to other structures such as the self-alignedstructure (SAS), electrode stripe structure, embedded hetero structure(BH), and channeled substrate planar (CSP) structure to achieve the sameeffects as described above without affecting the substance of theinvention.

According to the present invention, cleavage guide grooves are formed ina direction of cleavage near optical waveguides so that individualnitride semiconductor devices are obtained by being diced apart throughcleavage occurring along those cleavage guide grooves. This makes itpossible to reduce the surface roughness on end surfaces near theoptical waveguides of the nitride semiconductor devices and therebyobtain mirror surfaces. This helps improve the FFP of the emitted lightand reduce the rate of defects, permitting nitride semiconductor devicesto be fabricated with a yield rate of 90% or higher.

What is claimed is:
 1. A nitride semiconductor device comprising: asubstrate that cleaves along a cleavage plane; a nitride semiconductorlayer disposed on the substrate, having a cleavage planecrystallographically similar to the cleavage plane of the substrate,comprising a compound containing a group III element and nitrogen andhaving a cleaved end surface cleaved along the cleavage plane; acleavage guide groove portion formed in the nitride semiconductor layerfor forming the cleaved end surface; and a cavity comprising astripe-shaped optical waveguide formed in the nitride semiconductorlayer and the cleaved end surface of the nitride semiconductor layer,wherein the cleavage guide groove portion extends from a top surface ofthe nitride semiconductor layer and reaches the substrate, and thecleavage guide groove portion does not extend over a region of thenitride semiconductor layer above the stripe-shaped optical wave guide.2. A nitride semiconductor device as claimed in claim 1, wherein thesubstrate comprises a nitride semiconductor comprising a compoundcontaining a group III element and nitrogen.
 3. A nitride semiconductordevice as claimed in claim 1, wherein a depth from the top surface ofthe nitride semiconductor layer to a bottom of the cleavage guide grooveportion is between 1 μm and 10 μm.
 4. A nitride semiconductor device asclaimed in claim 3, wherein a lateral edge of the cleavage guide grooveportion is at least 50 μm away from a lateral edge of the stripe-shapedoptical waveguide.
 5. A nitride semiconductor device as claimed in claim1, wherein a depth from an interface between the substrate and thenitride semiconductor layer to a bottom of the cleavage guide grooveportion is between 1 μm and 10 μm.
 6. A nitride semiconductor device asclaimed in claim 5, wherein a lateral edge of the cleavage guide grooveportion is at least 50 μm away from a lateral edge of the stripe-shapedoptical waveguide.
 7. A nitride semiconductor device as claimed in claim1, wherein the cleavage guide groove portion is at least 50 μm away fromthe stripe-shaped optical waveguide in a direction perpendicular to thestripe-shaped optical waveguide within any plane parallel to a primaryplane of the substrate.
 8. A nitride semiconductor device as claimed inclaim 1, wherein a lateral edge of the cleavage guide groove portion isat least 100 μm away from a lateral edge of the stripe-shaped opticalwaveguide.
 9. A nitride semiconductor device as claimed in claim 1,wherein the nitride semiconductor device has a thickness within a rangefrom 80 μm to 160 μm.
 10. A nitride semiconductor device as claimed inclaim 1, further comprising a cleavage assist groove portion formed in abottom surface of the substrate.
 11. A nitride semiconductor device asclaimed in claim 10, wherein the substrate comprises a nitridesemiconductor substrate made of a compound containing a group IIIelement and nitrogen.
 12. A nitride semiconductor device as claimed inclaim 10, wherein a depth from the top surface of the nitridesemiconductor layer to a bottom of the cleavage guide groove portion isbetween 1 μm and 10 μm.
 13. A nitride semiconductor device as claimed inclaim 12, a lateral edge of the cleavage guide groove portion is atleast 50 μm away from a lateral edge of the stripe-shaped opticalwaveguide.
 14. A nitride semiconductor device as claimed in claim 10,wherein a depth from an interface between the substrate and the nitridesemiconductor layer to a bottom of the cleavage guide groove portion isbetween 1 μm and 10 μm.
 15. A nitride semiconductor device as claimed inclaim 14, wherein a lateral edge of the cleavage guide groove portion isat least 50 μm away from a lateral edge of the stripe-shaped opticalwaveguide.
 16. A nitride semiconductor device as claimed in claim 10,wherein a lateral edge of the cleavage guide groove portion is at least50 μm away from a lateral edge of the stripe-shaped optical waveguide.17. A nitride semiconductor device as claimed in claim 10, wherein alateral edge of the cleavage guide groove portion is at least 100 μmaway from a lateral edge of the stripe-shaped optical waveguide.
 18. Anitride semiconductor device as claimed in claim 10, wherein the nitridesemiconductor device has a thickness within a range from 80 μm to 160μm.
 19. A nitride semiconductor device comprising: a substrate thatcleaves along a cleavage plane; a nitride semiconductor layer disposedon the substrate, having a cleavage plane crystallographically similarto the cleavage plane of the substrate, comprising a compound containinga group III element and nitrogen and having a cleaved end surface alonethe cleavage plane; a cavity comnrising a stripe-shaped opticalwaveguide formed in the nitride semiconductor layer and the cleaved endsurface of the nitride semiconductor layer; and a cleavage guide grooveportion formed in a bottom surface of the substrate for forming thecleaved end surface the cleavage guide portion not extending over aregion of the substrate below the stripe-shaped optical waveguide.
 20. Anitride semiconductor device as claimed in claim 19, wherein thesubstrate comprises a nitride semiconductor substrate made of a compoundcontaining a group III element and nitrogen.
 21. A nitride semiconductordevice as claimed in claim 19, wherein a depth from the bottom surfaceof the substrate to a bottom of the cleavage guide groove portion isbetween 1 μm and 10 μm.
 22. A nitride semiconductor device as claimed inclaim 21, wherein a lateral edge of the cleavage guide groove portion isat least 100 μm away from a lateral edge of the stripe-shaped opticalwaveguide.
 23. A nitride semiconductor device as claimed in claim 19,wherein a lateral edge of the cleavage guide groove portion is at least50 μm away from a lateral edge of the stripe-shaped optical waveguide.24. A nitride semiconductor device as claimed in claim 19, wherein alateral edge of the cleavage guide groove portion is at least 100 μmaway from a lateral edge of the stripe-shaped optical waveguide.
 25. Anitride semiconductor device as claimed in claim 19, wherein the nitridesemiconductor device has a thickness within a range from 80 μm to 160μm.